CNG has recently been reevaluated as an alternative fuel for industrial engines burning gasoline, diesel or liquid propane gas fuels (LPG), especially with an increased emphasis being placed on all off-highway vehicle emissions. The highway vehicles, such as trucks, buses and automobiles have long been using CNG alternative fuels, but because of the fewer total miles or hours of operation, industrial vehicles, such as forklift trucks, have been exempt from the more stringent air quality standards that have applied to highway vehicles. Many workplaces, where off-highway vehicles are used, are now subject to those same standards.
CNG fuel produces substantially lower harmful emission products than gasoline, diesel or LPG. LPG-propelled lift trucks have long been preferred in a closed environment over gasoline or diesel fuel. However, emission tests have shown that the CO levels are 90% higher, and the reactive hydrocarbons (excluding natural gas or methane) are 70% higher in LPG than in CNG. Indoor applications such as in transport terminals and warehouses create difficult air quality conditions. While many warehouses are adequately ventilated, others are not. Trailers are loaded from the rear and when backed into the loading dock of the warehouse, they block off, for much of the time, fresh air which would otherwise enter the building. Environmental authorities, state and federal, are pressing for improvements to be made in the ventilation systems of buildings where powered equipment is used to meet even lower emission levels than is possible with LPG-propelled machines. The cost to industry of upgrading the ventilation system in existing structures is often prohibitive.
For economic reasons, it is expected that CNG as a fuel source will begin replacing, in many instances, the LPG-powered engines, especially in high density, long cycle operations of the type where lift trucks are used and where the existing ventilation system proves inadequate to lower the emission levels to that required by the stricter environmental standards, and upgrading the existing ventilation systems is more costly than converting the lift truck fleet over to an alternative CNG fuel system.
Typically, a single LPG tank for operating a lift truck will hold enough liquid propane to last a normal eight-hour shift, about 33 pounds, without being replaced with a fresh tank. Empty tanks are placed in a rack normally located outside the building, where they are picked up periodically by a service company for recharging.
Unlike propane, natural gas is delivered to the workplace by gas pipeline, since the same natural gas is used also for other purposes, such as heating the building. The local natural gas company will often install a fueling station for the natural gas customer and simply add the service fee to the monthly bill for natural gas which is metered out for the truck fleet as well as for the other natural gas consumption on the premises. A fueling station comprises a compressor, charging cylinders, a control panel and associated equipment for storing and dispensing natural gas under pressure. A station will typically have a standard 12 or 14-foot length of hose connected to a dispensing tower and manifold equipped with pressure gauges and shut-off valve for turning on and off the gas supply. A filling nozzle at the end of the hose is insertable into a refueling receptacle on the truck. The shutoff valve at the manifold is turned on at the dispensing tower and the filling nozzle on the hose inserted into the refueling receptacle of the truck. The nozzle is opened, allowing natural gas under pressure to flow from the dispensing tower into the tank on the truck.
Unlike propane, natural gas is lighter than air and even at high pressure (3000 psi at 70.degree. F.), it remains a gas. As a result, a CNG tank must have a thicker wall, reinforced, usually with wound glass strands. The wall thickness of the tank may be 1/4 inch or more. The specifications for such tanks are strictly controlled by the U.S. Department of Transportation (DOT). Tank manufacturers are issued permits by the DOT, and the manufacturing process is subject to periodic inspection, plus the tanks are required to undergo periodic pressure testing.
The outside dimensions of the CNG tank will be about twice those of the typical LPG tank to provide the equivalent amount of fuel capacity to operate a lift truck for a normal eight-hour shift. In terms of usable energy for the normal work cycle of the truck, 670 cu. ft. of natural gas at 3,000 psi and 70.degree. F. equates to about 28 pounds of propane.
One difficulty with CNG systems has been mounting the bulky tank in the limited space available outside the engine compartment on smaller machines, such as compact loaders and lift trucks.
Mounting CNG fuel components inside the engine compartment of a compact vehicle, which for reasons of cost, must ordinarily be designed for the higher unit sales of gasoline, diesel or LPG fuel trucks, is difficult to accommodate in the restricted space environment allowed for these fuel systems. The additional parts of the more complex CNG fuel systems has required mounting not only the tank, but high pressure lines and sensitive components outside the engine compartment. This leaves them exposed to being struck while the vehicle is moving by protruding objects or unstable loads, possibly causing a release of high pressure fuel.
Unlike the LPG fuel system, where the refueling operation comprises replacing the empty tank with a recharged one from a convenient storage rack, in the CNG system the tank remains mounted on the truck and is backfilled with gas under pressure delivered by a nozzle inserted in a refueling valve in the high pressure lines. Fuel supply is not totally interrupted during the refueling of the CNG system, as it is in the case of the tank replacement for an LPG system, hence, it is possible, despite cautions to the contrary, to refuel with the engine running.